Apex domains cannot have CNAME records.
Question and response
Question
dig interrisk.de. NS
Answer
interrisk.de. 1 NS ns1.vistec.net.
interrisk.de. 1 NS ns2.vistec.net.
Authority
The response has no authority records.
Additional
ns1.vistec.net. 1 A 213.216.11.47
ns2.vistec.net. 1 A 212.71.195.66
ns1.vistec.net. 1 AAAA 2a02:13c0:0:0:0:0:11:47
ns2.vistec.net. 1 AAAA 2001:14f8:1:7:0:0:1:11
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0